Outside of specific characterizations, I would say that the entire premise and exposition of the work reflects American life in the 1930s. Work is scarce, and individuals must travel far and wide in order to get it. This creates a setting where there is little in way of solidarity between people, and the desire to achieve a material standard of living supersedes all bonds of connection. The state of economic affairs dictates all and within this, individuals learn that affection and collectivity are distant second to material happiness. Within this harshly deterministic configuration, individuals still hold on to their dreams of farms and animals or of ball games and simple joys and their navigation between both the realistic condition of what is and the hopeful vision of what can be formed the backdrop of American life at the time.
Steinbeck's novel, Of Mice and Men, reveals a perfect cross section of 1930's Depression-era America. There is Candy - the senior citizen who is left out of any chance at a secure retirement.
Lennie, who is one of the mentally handicapped who falls through the cracks of a society with no plan to care for him, and George, the mostly unemployed migrant worker who just wants a little peace of mind and stability in an economically uncertain time.
Then consider Curley's wife, who represented the conservative society that expected women to stay home and raise children, with few other options.
Numerous characters and events in Of Mice and Men might represent life in the thirties in America, as well as life in other periods and places. One element in the novel that, while it certainly does apply to America before and after the thirties, most specifically represents the thirties (due to the economic Depression ) is economic injustice/class conflicts. The enotes Study Guide on the novel says:
Although George and Lennie have their dream, they are not in a position to attain it. In addition to their own personal limitations, they are also limited by their position in society. Their idealistic dream is eventually destroyed by an unfeeling, materialistic, modern society. The tensions between the characters are inherent in the nature of American capitalism and its class system. Curley, the son of the ranch owner, is arrogant and always looking for a fight. This is not merely a personality trait. His position in society has encouraged this behavior; his real strength lies not in his fighting ability but in his power to fire any worker. Similarly, Carlson, the only skilled worker among the ranch hands, is arrogant and lacks compassion. Carlson would be difficult to replace in his job as a mechanic; therefore, he feels secure enough in his status to treat the other workers sadistically. This trait is seen when he orders Candy's dog to be shot and when he picks on Lennie. The other workers go along with Carlson because they are old or afraid of losing their jobs. Lennie's mental retardation also symbolizes the helplessness of people in a capitalistic, commercial, competitive society. In this way, Steinbeck illustrates the confusion and hopelessness of the Depression era. The poor were a class of people who suddenly had captured the imagination of American writers in the 1930s. This was an example of the shift in attitudes that occurred during the Depression. Previously, American fiction had been concerned with the problems of middle-class people. Steinbeck's novel was a sympathetic portrayal of the lives of the poorest class of working people, while exposing society's injustices and economic inequalities in the hope of improving their situation.
Notice, for instance, the poignant scenes in which George and Lennie talk about raising a stake, working and saving, as well as about raising rabbits. They do have personal shortcomings that make the fulfillment of their dreams highly unlikely, but shortcomings or not, their positions in society, and the nature of American capitalism, pretty much doom them to the place in society they have in the novel. Of Mice and Men reveals this.